The announcements service can generate events like so (with an event of "annc.null"):
EVENT_ID: 950053626
EVENT_DATE: 2013-07-19 15:12:45
EVENT: annc.null
REF: /announcement/msg/222222/main/f83872cc-e4d0-4389-93ee-1af7294492b0
CONTEXT: 222222
SESSION_ID: XXXXXXXXXXXXXXXXXXXXXX
EVENT_CODE: m

This is because of a method called eventId which is called like so:

groupRefs = m_authzGroupService.getAuthzGroupsIsAllowed(m_sessionManager.getCurrentSessionUserId(),
eventId(function), groupRefs);

That eventId method looks like this:
protected String eventId(String secure) {
return SECURE_ANNC_ROOT + secure;
}

The method does not check for or handle properly invalid inputs.